Preheat the oven to 375°F. Line an 8-inch sponge cake pan with parchment paper.
Combine the butter and milk together in a saucepan over medium heat. Cook, stirring, until the butter melts. Remove from the heat and set aside.
In a mixing bowl, add the egg whites and beat with an electric mixer until stiff peaks form. Add the yolks and continue to beat. With the mixer running, slowly pour in the sugar and mix until incorporated. Fold in the cream of tartar, salt, and flour.
Add the baking soda to the milk mixture and stir well then fold into the batter. Pour the batter into the prepared pan. Place in the oven and bake at 375°F for 30 minutes or until the cake springs back when touched.
Remove from the oven and let cool for 5 minutes in the pan then turn out onto a cooling rack.